The present invention relates to a fuel metering system, and more particularly to a fuel metering system having a planar diaphragm for an externally-purged-type carburetor.
Typically, carburetors have been used to supply a fuel-and-air mixture via an intake passage to both four stroke and two-stroke internal combustion engines. For many applications where small two-stroke engines are utilized, such as hand held power chain saws, weed trimmers, leaf blowers, garden equipment and the like, carburetors with both a diaphragm fuel delivery pump and diaphragm fuel metering system have been utilized. When the engine is operating, the diaphragm fuel delivery pump supplies fuel under pressure to the diaphragm fuel metering system through an inlet or flow control valve of the fuel metering system, which in-turn supplies fuel to a fuel-and-air mixing passage of the carburetor for mixing with air prior to flowing into a combustion cylinder of the engine.
A convoluted flexible diaphragm or membrane of the fuel metering system typically has a peripheral edge sealed to the carburetor body. A metering chamber and an air chamber is thus partitively disposed over and under the diaphragm, respectively. During operation, when the amount of fuel in the chamber decreases and the convoluted diaphragm is moved due to a negative pressure in the fuel-and-air mixing passage, the flow control valve is opened against the force of a spring by a pivoting lever that operates together with the diaphragm and is fixed to a wall of the carburetor body by a support shaft. In this way, the fuel is supplied from the fuel delivery pump to the metering chamber. As a result, the amount of fuel in the metering chamber is kept at about a constant level or volume.
Commonly, the carburetor has an external purge or manually actuated primer or suction pump having a flexible bulb attached to the bottom side of the carburetor body. The bulb internally defines a pump chamber in which a composite valve functions to admit fuel to the pump chamber and deliver fuel to the metering chamber of the fuel metering system. Moreover, before the engine starts for operation, the bulb is repetitively manually pressed and released to suck unwanted fuel vapor and air from the fuel pump and fuel metering system into the pump chamber of the external purge via the composite valve. The fuel vapor and air are transferred back to the fuel tank via the composite valve. At this time, since the metering chamber is under a negative pressure, the fuel in the fuel tank is supplied to the metering chamber through a fuel chamber of the fuel delivery pump and the flow control valve.
The diaphragm of the fuel metering system typically has five basic functions: (1) maintain a seal between the air and the metering chambers, (2) respond instantly to differential pressure (engine manifold pressure referenced to atmospheric), (3) open the flow control valve when the engine needs fuel, (4) close the flow control valve when the engine has enough fuel, and (5) perform consistently over the life of the engine (i.e., no loss of elastomeric flexibility of the convoluted diaphragm from age or fuel exposure).
The convoluted metering diaphragm is typically made of an elastomeric membrane and molded to form convolutions to achieve flexibility and a pre-established total travel distance necessary to open and close the flow control valve. This total travel distance commonly ranges from about 0.020 to 0.065 of an inch, and includes a degree of free-play before a head of the flow control valve actually moves to open and close the valve. During engine operation, from idle to wide open throttle conditions, the convoluted diaphragm typically moves approximately within a range of 0.001 to 0.015 of an inch and thus the head proportionately moves accordingly. This range depends upon the carburetor and its application. FIGS. 8-10, illustrated as prior art, show such a metering diaphragm 20 having a molded convolution 22. Under normal engine/carburetor operating conditions, a center or circular section 24 of the diaphragm, circumscribed by the convolution 22, provides the primary movement for operation of the flow control valve 26. The convolution itself has little contribution to achieving the required fuel delivery pressure balance in the metering chamber (not shown). The metering diaphragm 20 transmits a relative movement to a pivoting lever 28 which transmits opposite movement to a head 30 of the flow control valve 26 based on a pressure differential formed across the diaphragm. The differential is initiated from the sub-atmospheric pressure exposed to the metering chamber by the fuel-and-air mixing passage of the carburetor and the reference atmospheric pressure of the air chamber of the metering system.
FIGS. 8 and 9 illustrate the common convoluted metering diaphragm 20 having a central rigid plate 32, a washer 34 and a rivet button 36 for transmitting this force to the pivoting and spring biased lever 28 of the flow control valve 26, which in turn moves the valve head 30 away from a valve seat 38 carried by the carburetor body to open, and against the valve seat 38 via the resilience of the spring (not shown) to close the valve. The diaphragm must have sufficient resilience for transmitting displacement in proportion to the pressure differential, yet remain flexible enough to respond to sudden changes in pressure such as for engine acceleration and engine starting. Unfortunately, the cost of manufacturing a flexible diaphragm having rigid hardware which is engaged sealably to the diaphragm is expensive, and the diaphragm penetration required to secure the hardware creates a source of potential leakage between the metering chamber and the reference chamber.
Aside from the rigid hardware, there are several reasons for the additional diaphragm travel afforded by the convolution in a standard diaphragm carburetor design. The convolution provides extra material for maintaining diaphragm flexibility should the fabric or elastomer coating shrink (typically made of woven silk and nitrile material) upon exposure to hydrocarbon fuels or aging effect. This extra material measured or extending perpendicular to the general plan of the diaphragm itself also maintains necessary operating clearances or free-play travel distance between the pivoting lever and diaphragm if this shrinkage occurs. The extra convolution material also allows more diaphragm travel (increased metering fork leverage) to xe2x80x9cuncorkxe2x80x9d a stuck head of the flow control valve, particularly for carburetors which do not have a manual external purge or bulb device to create a strong vacuum. In-other-words, the convolution assists to release stuck heads for those carburetors which utilize the weaker engine manifold vacuum in combination with a choke valve to generate the metering chamber vacuum for opening the flow control valve for purging the carburetor of air or vapor to better start the engine.
However, there are also inherent problems associated with the metering diaphragm convolution which have adverse impact on carburetor performance. Such problems include the inadvertent changes in baseline carburetor fuel flow settings, inconsistent fuel delivery and exhaust emission variation, poor acceleration response, and the potential for leaking/dripping from the carburetor main nozzle. For instance, a distorted convoluted diaphragm can change the original or installed operating clearance between the rivet button and the lever so that an adverse shift in idle performance due to vibration or orientation of the engine can cause fuel leakage leading to a rich idling engine. At wide open throttle conditions, such fuel leakage can result in engine stall during deceleration from wide open throttle to idle. For non-running engines, a distorted convolution which eliminates clearance can depress the lever to allow fuel leakage out of the carburetor causing fuel tank drainage.
The process of convolution molding is known to contribute to variations in diaphragm flexibility based on molding temperatures and pressures, and aging which is also influenced by the composition of the elastomeric material and substrate fibers. Natural cotton or silk substrates have been used historically for flexibility and elastomeric bonding, but these natural fibers in combination with a molded convolution are susceptible to hygroscopic absorption leading to uncontrolled changes in convolution height influenced by ambient humidity which directly adversely impacts the operating clearance. Use of nylon or other synthetic polymers in lieu of natural fibers as the substrate material for the molding process to create the convolution may contribute to additional molding stress and memory set of the convolution resulting in diaphragm rigidity and inconsistent response to small differential pressures. Thickness variation of the elastomeric coating and its cured state also contribute to poor diaphragm response and flexibility changes through molding the metering diaphragm convolution. Pin holes or elastomer tears can occur at the base of the convolution during the molding process where the base material is squeezed and stretched under heat and pressure, leading to potential fuel and/or air leaks across the metering diaphragm.
In addition, residual stresses from both the molding process and fabrication of the diaphragm material can be accentuated upon exposure to hydrocarbon and aromatic compounds in the fuel causing diaphragm convolution distortion or changes in material property. For example, conventional Nitrile rubber compounds can lose plasticicizers blended in the rubber from fuel leachment breaking the elastomeric chemical bonds resulting in adverse stiffness affecting flexibility characteristics of the convoluted metering diaphragm. Other types of elastomeric and substrate materials may also exhibit various degrees of swell, shrinkage, and flexibility characteristics exacerbated by the convolution which alter the ability of the diaphragm to respond consistently and repeatably to small pressure differentials.
Specific convolution anomalies involving convoluted metering diaphragms include variation in convolution datum height affecting lever/diaphragm clearances, non-symmetric convolution axis or distorted convolution affecting diaphragm pressure response and recovery, oil canning of the diaphragm during flexure causing erratic diaphragm movement, fuel and air leakage across the diaphragm from holes or tears or poor elastomeric coating processes. These examples contribute inconsistent carburetor fuel flow settings, poor engine acceleration, engine stalls during rollout, hard starting, and fuel leakage/flooding. It becomes more of a prevalent problem on those engine applications with relative weak manifold vacuum, lean carburetor setting for lower exhaust emissions, or large frictional differences in the engine (new versus broke-in engine) which make the carburetor more sensitive to variation in diaphragm flexibility.
A fuel metering system for a combustion engine carburetor utilizes a non-convoluted, planar, flexible diaphragm which does not require a molding process to form a traditional convolution. The diaphragm defines in part a fuel metering chamber on one side and a reference chamber at near atmospheric pressure on the other side. During operation of the engine, sub-atmospheric pressure within a fuel-and-air mixing passage draws fuel from the metering chamber to mix with air for combustion within the engine. As pressure within the metering chamber thus decreases, the diaphragm flexes into metering chamber. The displacement of the diaphragm actuates a flow control valve of the metering system which flows pressurized make-up fuel into the metering chamber until the diaphragm returns to its datum position. Preferably, hardware of the flow control valve which is in direct contact with a surface of the diaphragm exposed to the metering chamber does not require penetration of the diaphragm, as the traditional rivet and washer assembly does. Therefore, manufacturing costs are reduced and any opportunity of leakage between the fuel metering chamber and reference chamber is eliminated. Preferably, the carburetor is of a manual external purge type in order to exert sufficient vacuum within the metering chamber to displace the planar metering diaphragm thus opening the flow control valve to purge the carburetor of unwanted fuel vapor and air prior to starting the engine. The novel planar diaphragm thereby resolves problems associated with traditional convoluted metering diaphragms such as the variation in convolution datum height affecting flow control valve lever/diaphragm clearances, and non-symmetric convolution axis or distorted convolution affecting diaphragm pressure response and recovery.
Preferably, in order to achieve the flexibility and fuel absorption resistance necessary for the unique operating characteristics of the flat metering diaphragm, the traditional composite material of nitrile and silk fabric is replaced with a a synthetic woven fabric impregnated with a synthetic rubber, such as nylon and nitrile. The nylon fabric has extremely small diameter fiber bundles in the weave providing increased flexibility with favorable recovery characteristics (return to datum position upon removal of differential pressure across the diaphragm). In addition, the elastomeric composition is such that fuel permeability is decreased when compared to that of typical diaphragm materials used in the past. This decrease in fuel permeability is favorable for emission control requirements. Moreover, the synthetic rubber and fabric combination preferably has a surface texture and elastomeric properties conducive to minimal abrasion wear. This is necessary for the preferable novel flow control valve lever of the present invention which must act directly upon the metering diaphragm in both wet and dry environments.
Objects, features and advantages of this invention include a metering diaphragm which is non-convoluted eliminating the convolution height variations created in manufacturing, diaphragm fuel absorption and aging of the traditional diaphragm which adversely affects flow control valve and thus engine operation. Moreover, leakage between the metering and air chamber is eliminated via the novel flow control valve lever of the present invention thereby providing a reliable smooth running engine. Additional advantages are a reduced number of parts, reduced number of manufacturing processes, and a design which is easily incorporated into existing carburetors. This design improves engine performance and is relatively simple and economical to manufacture and assemble, and in service has a significantly increased useful life.